We argue that the distinguishing features of Participatory Designare not the participatory activities as such but the mechanisms used,the effects produced and the way in which these are sustained. Weuse program theory to illuminate how participatory design worksand how it may be understood as more than a collection of methods or a matter of configuring user participation. Program theoryoperates by formulating the causal relations between the plannedinputs, the process and the effects in terms of output, outcomeand impact. While participatory design might appear similar toco-design or user-centered design on the level of design activities,PD differs in terms of the mechanisms employed and how effectsand their sustainment are conceived. Looking at participatory design through the lens of program theory highlights how particularmechanisms work towards not only the designed product but alsotowards generating gains and lasting effects for participants.